1. Technical Field
The present invention relates to a floor structure for a truck's load-carrying platform, and more particularly to a floor structure which is applicable to any type of load-carrying platform such as a van's platform (i.e., closed box-type platform), a platform which opens in three directions (e.g., a pick-up truck's platform), a platform equipped with a single gate which opens in a backward direction and a wing-body type platform which has one or more pairs of swing doors adapted to open like bird's wings in a width direction of a truck.
2. Background Art
Referring to FIG. 35 of the accompanying drawings, a wing body-type platform equipped with a single pair of swing doors or roofs 202 is illustrated as an example of typical truck's load-carrying platforms. This platform has relatively large dimensions and fixed on a frame work of a truck. The platform has gates 201 along lateral edges of the truck and the wing roofs 202 over the gates. At a rear end of the platform, provided is a pair of biparting rear doors 203. A front end of the platform is closed by a front wall 204. It should be noted that a cab (not shown) is situated in front of the front wall 204. The gates 201, wing roofs 202, rear doors 203 and front wall 204 are supported from a conventional floor structure 205 of the truck's platform respectively.
Referring to FIG. 36 of the accompanying drawings, illustrated is a perspective view of part of the floor structure 205 in an enlarged scale partly in cross section. The floor structure 205 is secured on a pair of side frames 206 (only one of them is depicted) of a ladder-like truck frame via spacers 207 by suitable fastening members such as U-bolts (not shown). The side frames 206 extend in a longitudinal direction of the truck. The floor structure 205 includes longitudinal sills or joists 208 extending along the side rails 206 and secured on the side rails 206, transversal sills 209 extending in a direction perpendicular to the longitudinal sills 208 across the width of the platform and secured on the longitudinal sills 208 and the floor plate 210 placed on the transverse sills 209. The floor plate 210 forms a platform floor. Each of the longitudinal and transversal sills 208 and 209 is a steel member having a U-shaped cross section. The longitudinal and transversal sills 208 and 209 form in combination a frame work to support the floor plate 210 which is situated on the frame work. The rigidity of the floor structure 205 is insured by this frame work. The floor plate 210 is generally made of wood.
As understood from FIG. 36, the conventional floor structure 205 is made by the longitudinal and transversal sills 208 and 209 assembled like a lattice and the floor plate 210 placed on them. Therefore, the floor structure 205 has a thickness Tz which is relatively Large. On the other hand, there is a strong demand for low floor trucks in an automobile industry. Loading and unloading becomes easier as the height of the platform floor from the ground becomes lower. In addition, the lower the floor, the larger a volume of the platform.
The conventional floor structure 205 is also heavy in weight since it includes the steel frame members 208 and 209 and the woody floor plate 210.
The floor structure 205 itself must have sufficient rigidity or strength since it helps insure or partly determines the truck's overall bending/flexural rigidity as well as torsional rigidity.
In general, a typical truck has wheels below the platform floor and outside the side rails of the truck's frame, and the wheels are frequently caused to move up and down due to projections and recesses on and in a road surface and/or cornering (or rolling) movements of the truck. Therefore, if the floor structure of the truck's platform is positioned at a lower height from the ground or road surface, the wheels may collide with the floor panel when the wheels move upward.